I have four identical Hampton Bay Ceiling Fans. One of them works on low speed only. I replaced the four wire fan switch, but it did not correct the problem. Thanks!

Are they on remote control's or rheostats? If so, you will need to pull the chain until it is set on high. Then Hi/Med/Low will work.

Are these pullchain fans or remote fans? Sounds like a bad capacitor. See:

Ceiling fan capactors - troubleshooting and resources - Ceiling Fans N More